The Collaborative International Dictionary of English v.0.48:

Corsair \Cor"sair\ (k?r"s?r), n. [F. corsaire (cf. It. corsare,
   corsale, Pr. corsari), LL. corsarius, fr. L. cursus a
   running, course, whence Sp. corso cruise, corsa cruise,
   coasting voyage, corsear to cruise against the enemy, to
   pirate, corsario cruising, a privateer authorized to cruise
   against the enemy. See Course.]
   1. A pirate; one who cruises about without authorization from
      any government, to seize booty on sea or land.
      [1913 Webster]

   2. A piratical vessel.
      [1913 Webster]

            Barbary corsairs . . . infested the coast of the
            Mediterranean.                        --Prescott.
      [1913 Webster]

   3. (Zool.) A Californian market fish (Sebastichthys
      rosaceus).
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

corsair
    n 1: a pirate along the Barbary Coast [syn: corsair, Barbary
         pirate]
    2: a swift pirate ship (often operating with official sanction)

The Devil's Dictionary (1881-1906):

CORSAIR, n.  A politician of the seas.